Copper sheets, aluminum sheets, atmospheric corrosion resistant steel sheets, stainless steel sheets, zinc alloy sheets, galvanized steel sheets, etc. have been conventionally used as metallic materials for roofing and facing in building.
Each metallic material has its advantages and disadvantages and these materials are selected in accordance with the intended use. Copper sheets, aluminum sheets, atmospheric-corrosion resistant steel sheets, stainless steel sheets, zinc alloy sheets and galvanized steel sheets have the following advantages and disadvantages.
Copper develops green rust (verdigris, a basic carbonate salt) on its surface. The tint of this rust imparts elegant appearance to the edifice and, therefore, copper has been used for Shinto shrines and Buddhist temples in Japan from olden times. However, copper causes galvanic corrosion of other metals that are used in combination with it. Aluminum, iron and zinc, which are baser than copper, corrode in the presence of copper. Also copper ions which are formed and washed out by rain water may stain the underlying materials. Further, the toxicity of copper ions may kill nearby plants. Among metals, copper is rather soft and, therefore, this material cannot be used in applications in which strength is required. Thus, steel sheets which are plated with copper are sometimes used. This material, of course, suffers from galvanic corrosion.
Aluminum is a very base metal but corrosion resistance is ensured by the oxide film which forms on the surface. However, it may suffer serious pitting depending upon the conditions in which it is used. Corrosion of aluminum starts from the points where dust, iron powder or chlorine ions adhere and, therefore, occasional cleaning is required. Frequent cleaning is necessary in seashore regions or heavily polluted places. Thus aluminum cannot be used for parts used where cleaning is difficult.
The corrosion resistance of atmospheric-corrosion resistant steel sheets is maintained by the dense rust formed on the surface by virtue of the alloying elements. Corrosion of atmospheric-corrosion resistant steel sheets starts from defect points of this surface rust and the produced red rust stains concrete and other materials and spoils the appearance of buildings.
Corrosion resistance of stainless steels is based on the passive films formed on the surface thereof. However, stainless steels often suffer from pitting or crevice corrosion, which produces red rust and spoils the appearance of buildings. Shining appearance of stainless steels does not harmonize with natural environments and, therefore, they are sometimes painted or subjected to other surface treatment.
Zinc is a base metal which is highly corrodible, However, it maintains its atmospheric-corrosion resistance by virtue of a basic corrosion product which forms on the surface. Zinc. protects other metals by sacrificial corrosion and thus is used for plating steel sheets. Corrosion of zinc produces white rust. Zinc has a larger expansion coefficient than other metals and, therefore, its use is restricted in environments where the temperature difference between day and night and between summer and winter is great. Also, zinc is very soft and its use is limited in the condition where strength is required. Therefore, zinc is used in the form of zinc alloys which are strengthened by alloying elements or zinc-plated (galvanized) steel sheets. However, galvanized steel sheets are not sufficient in corrosion resistance in some applications and suffer from formation of corrosion holes and generation of red rust.
These copper sheets, aluminum sheets, atmospheric-corrosion resistant steel sheets, stainless steel sheets, zinc alloy sheets and galvanized steel sheets are usually used as is. However, recently there is a trend in which coloring or decoration is desired in the exterior use and they are painted or given some surface treatment in such cases.
As has been described, copper, aluminum, atmospheric corrosion resistant steels, stainless steels, zinc, galvanized steel sheets are used in accordance with their advantages and disadvantage for intended use.
The properties required for roofing and facing materials are summarized as follows.
(1) To have sufficient atmospheric-corrosion resistance and minimal change in appearance (fading of color, reduction in luster, corrosion) under the environment in which they are used. PA1 (2) To have sufficient strength as roofing and facing materials. PA1 (3) To have a small expansion coefficient. PA1 (4) To have a color harmonizing with the environment in which they are used.